Bottlehead Forum
General Category => Technical topics => Topic started by: CrowDaddy on June 12, 2023, 08:39:52 PM
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Have the gang at Bottlehead thought about the circuits Menno Vanderveen has published in audioxpress?
Seems that a reduction in effective plate resistance, and a reduction in magnetic distortion in the OT, would be worthwhile. Have you had a chance to listen to his SE10, for example?
His recent designs use solid-state drivers to produce the current input for the output stage, but tube drivers should be able to do this as well.
Peripheral question: he points out that this technique was first published in 1940. Do you know of any vintage amps which used it?
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We had a kit called the Seductor that used plate-to-plate feedback between the output tube and the driver tube in the same manner as the SE-10 (without the cathode follower).
I have used this in a dozen or so prototype transmitting tube amplifiers, and we are chipping away at a design to scale this up to offer quite a bit more power.
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How about Menno’s statement that a SE pentode gives twice the power of a triode, with an (arguably) preferable distortion spectrum?
Perhaps it takes his OTs to get that spectrum…
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Sort of true. In practice, it's usually a bit less that twice - pentodes are more efficient. You can get the same efficiency from a triode if you push enough grid current. In both cases, the source impedance is too high to be useful unless you use plenty of feedback. Vandeveen uses plate-to-grid feedback around the output stage, making it behave as a current-controlled voltage source - called a transresistance.
The driver acts as a voltage-controlled current source - called a transconductance. Vanderveen adds a local feedback to make that more linear. His driver cathode follower also is a feedback stage.
Lots of feedback means you need lots of open-loop gain, and therefor more tubes - the Vanderveen amp has four elements compared to two in most SETs. That eats up the cost savings from increased efficiency. No free lunch, no magic bullets.
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How about Menno’s statement that a SE pentode gives twice the power of a triode, with an (arguably) preferable distortion spectrum?
Perhaps it takes his OTs to get that spectrum…
The distortion behavior pushes more toward a solid state amp, where you get a sharper uptick in THD as you approach hard clipping, but not a lot before. The actual composition of the THD in such an amp is different from a typical SET which makes these circuits sound different. If your system and your listening tastes can make use of the extra power, it's a great way to go. If you're listening only requires 5W or so of power, a 300B is still a better choice IMO.